1. In a small bowl, whisk cornstarch into one-fourth-cup whipped topping. Set aside.
2. In a medium saucepan, bring one-fourth-cup whipped topping, champagne, one-fourth-cup sugar and confectioners’ sugar to a boil over medium heat. Stir and turn down heat to simmer.
3. In a separate bowl, whisk egg and egg yolks together until pale yellow. Add cream and cornstarch mixture. Pour one-fourth-cup of the hot champagne and cream mixture into the egg and cream mixture. Whisk it constantly so the hot mixture does not cook the egg mixture.
4. Pour that mixture back into the saucepan. Whisk over medium-low heat until it thickens and coats the back of a spoon. As soon as it is thickened, remove from heat and whisk in margarine and vanilla extract.
5. Allow pastry cream to cool slightly and cover with plastic wrap pressed onto the surface of the custard so it doesn’t form a skin. Cool completely.